Javascript, outside variable scope in callback function?

Question

everyone.

I have a problem about the 'callback function - scope of variable',

I wanna use the 'i in for loop' to 'the callback function User_UidSearch',

but I cannot use it.

(I hope the solution don't use the global variable.)

Task_TidSearchExecution = function(tid, callback) {
    var sql = "SELECT * FROM execution WHERE task = '" + tid + "'";
    dbclient.query(sql, function (err, results) {
        if (err || results.length <= 0)
            callback(false);
        else {
            console.log(results.length);
            for (var i = 0 ; i < results.length ; i++) {
                User_UidSearch(results[i].employee, function (user) {
                    console.log(i);
                    // results[i]['email'] = user.email;
                });
            }

            callback(results);
        }
    });
}

the "console.log(i);"

Recheck, this is wrong. -> Outputs are "undefined" of all.

undefined is "console.log(result[i]);"

But the "i" is keep '2' console twice, if results.length is 2.

I know becuz the for loop ending then execute the User_UidSearch,

but how can I slove the it "i" is 0 and 1.

No, it's not. It's defined in outer `function` local scope. But it's used in closure, which needs special logic to work. @Salmon - are you sure `console.log(i)` writes `undefined` and not the value of `results.length`? — Ilya Luzyanin, Aug 13 '14 at 07:55
"*Outputs are 'undefined' of all*" doesn't really fit with the snippet provided. `i` should at least be a number, just maybe not the particular values expected. If each log shows the value of `results.length`, then see: [JavaScript closure inside loops – simple practical example](http://stackoverflow.com/questions/750486/javascript-closure-inside-loops-simple-practical-example). — Jonathan Lonowski, Aug 13 '14 at 07:56
wrap the user_UidSearch call in a closure, pass in i and results — MartinWebb, Aug 13 '14 at 07:57
@IlyaLuzyanin: rigt.my bad i didnt see it through clearly.removed my comment — Prabhu Murthy, Aug 13 '14 at 07:58

score 3 · Answer 1 · edited May 23 '17 at 10:29

Your problem may be solved, but let me still add these points so others know the correct approach.

Firstly, its not a good practice to call an asynchronous function inside a loop. This would lead to unexpected results.

Using closures is fine for some cases, but not this one. Read here for Practical uses of closures.

There are several problems in the approach you've taken,

If the results array is too long, there'll be a too many open requests.
callback(results) will be called before even a single callback of User_UidSearch is called.

In your case, you should be using recursive function like this:

var len = results.length;
var count = 0;
function my_func() {
    User_UidSearch(results[count].employee, function (user) {
        console.log(count);
        // results[count]['email'] = user.email;

        count += 1;
        if (count < len) {
            my_func();
        } else {
            callback(results);
        }
    });
}

Read this if interested detailed explaination of different control flows.

Yes, you're right. I had solved your saying. But your solution is better than mine, thank you very much :) — Salmon, Aug 13 '14 at 09:29

score 1 · Accepted Answer · answered Aug 13 '14 at 08:41

1

You're dealing with closures, rewrite your code as follows:

...
(function(id) {
    User_UidSearch(results[id].employee, function (user) {
        console.log(id);
        // results[i]['email'] = user.email;
    });
})(i);

I.e. wrap your function to unbind i.

answered Aug 13 '14 at 08:41

Ilya Luzyanin

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Thank you very much, I follow this way solving. – Salmon Aug 13 '14 at 08:44
You're welcome! Closures are great stuff if you know how to use them right! – Ilya Luzyanin Aug 13 '14 at 08:45

Javascript, outside variable scope in callback function?

2 Answers2